Optical interconnect modules with 3d silicon waveguide on soi substrate

ABSTRACT

An active optoelectronic optical engine layer is assembled on a SOI silicon substrate side, including multiple VCSEL input ports and multiple PD output ports. Each passive straight silicon waveguide terminated with two 45 degree reflectors is fabricated on the silicon-on-oxide surface of the SOI substrate to form an optical path to transmit optical signals from the VCSEL lasers to photodetectors. The active and passive sides are designed and developed as an on-chip optical interconnect module to carry high-speed electrical traces, Improved RF performance is achieved. A method of fabricating the device is disclosed as: on the substrate surface, patterning a plurality of conductive lines and the optical engine has an emitter (VCSEL) and a receiver (PD); on the SOI device layer, patterning a straight portion and two 45 degree end reflectors as the silicon waveguide and align the two 45 degree end reflectors with active optical devices VCSEL and PD.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Patent Applications No. 62/508,940, titled “OPTICAL COUPLING STRUCTURE” filed on May 19, 2017, and No. 62/509,892, titled “OPTICAL INTERCONNECT MODULES” filed on May 23, 2017, which are incorporated by reference herein in their entirety.

FIELD

Embodiments described herein relate to optical interconnect modules, and more particularly optical sub-assembly systems in optical interconnects.

BACKGROUND

Cloud computing, enterprise networks, and data center networks continue to drive increased bandwidth demand of optical waveguides for metro and long haul wires, and also rack-to-rack wires within data centers to 100 Gbps and beyond. Increased bandwidth demand has motivated overall high data transmission speed on entire optical systems.

Optical interconnect techniques continue to gain attention as potential solutions for high-speed data transmission between systems, and over a variety of distances. For example, optical interconnect solutions have been proposed for a number of applications, such as between racks in a data center, between household consumer electronics, and between boards or chips within server systems. Optical interconnects are particularly suitable for adoption within transmitter and receiver systems.

In a conventional optical sub-assembly (OSA) design, a transmitter module incudes a transmission laser, a driver integrated circuit (IC), and a printed circuit board (PCB), while a receiver module includes a photodetector (PD), a trans-impedance amplifier (TIA), and a PCB. The optical path between the transmission laser (commonly a vertical cavity surface emitting laser (VCSEL)) and PD is typically an optical fiber, such as a fiber ribbon and optical waveguides. Complex beam routers including a focusing lens, a prism, and a fiber connector are used to precisely align the optical fiber with the optical path. Mechanical structures including screws, clips, alignment pins and structural housing are commonly used to secure and align the beam routers.

However, an optical interconnect typically requires coupling of fiber assembly and lasers which involves an external lens alignment, adding complexity and energy loss. A less complicated assembly technique is needed to improve efficiency and reduce cost.

SUMMARY

The application discloses a SOI substrate-based optical interconnect module. A 3-D optical waveguide is employed to connect the optical signal between transmitting and receiving parts.

An active optoelectronic package layer on bond pads is assembled on a SOI silicon substrate side. A passive straight silicon waveguides terminated with 45 degree reflectors is fabricated on the silicon-on-oxide surface (aka device surface) of the SOI substrate to form an optical path to transmit optical signals from multiple VCSEL input ports to multiple PD output ports. The active and passive sides are designed and developed as an on-chip optical interconnect module.

Active devices such as VCSEL, PD, Driver, and TIA arrays are flip-chip assembled via electrical traces and solder bumps on the silicon substrate side of above-mentioned SOI substrate. High-speed electrical traces, are designed and developed on the silicon substrate layer to connect the driver IC and VCSEL chips as well as to connect PD and TIA arrays. Improved RF performance of active devices by depositing electrical transmission lines on the SOI substrate. The data rate of above-mentioned optical interconnect modules can be operated at 25 Gbps per channel and can be extended as high as 50 Gbps per channel.

The silicon waveguides terminated with 45 degree reflectors are realized on (100) silicon device layer using chemical wet etching. The side walls of waveguide are formed on (110) oriented crystal facets.

The 3D optical path can be a basic and compact configuration in on-chip optical interconnect application.

The Driver and TIA arrays can be realized using CMOS fabrication or hybrid integration on the rear side of silicon substrate.

The VCSEL array can be fabricated on the front side of the SOI silicon substrate and the PD array can be realized using Si—Ge fabrication or hybrid integration on the rear side of silicon substrate.

A method of fabricating an optical interconnect apparatus, comprising: providing a silicon substrate having two substantially parallel first and second surfaces; depositing an insulating layer on the first surface; patterning a plurality of conductive lines on the insulating layer; growing bonding pillars or solder bumps with gold or nickel coating for flip chip to mount active optical devices such as a vertical cavity surface emitting laser (VCSEL) and a photodetector (PD); turning the silicon substrate over to work on second surface; patterning a straight portion of the silicon waveguide on the second silicon surface; patterning two 45 degree end reflectors on the straight portion of the silicon waveguide, wherein the patterning comprises variable exposure intensity in lithography process, wherein the two 45 degree end reflectors are aligned with active optical devices VCSEL and PD; removing excess silicon outside the silicon waveguide to expose the oxide layer; and depositing a layer of oxide on the waveguide. This method may include depositing an adhesive layer on the first surface by providing an anisotropic conductive film (ACF). The method of patterning the plurality of conductive lines may include two ways, 1) forming trenches and filling in metal in the trenches, followed by removing excess metal by polishing (CMP) or by selective etch to clean up metal outside the trenches, or 2) depositing a metal layer, patterning the metal layer into lines, followed by removing excess metal by polishing (CMP) or by selective etch. In summary, the disclosed configuration separates the active electronic-devices on silicon substrate layer and passive optical-devices on silicon device layer of SOI. The active devices can be realized by CMOS fabrication or hybrid integration when germanium device is included. The passive devices can be realized by MEMS (non-COMS) fabrication.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described some embodiments in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale.

FIG. 1 illustrates a SOI substrate.

FIG. 2 shows a preparation step for a waveguide on the silicon device surface of SOI substrate in accordance with an embodiment.

FIG. 3 shows another preparation step in forming the 45 degree reflector of the waveguide on the silicon device surface of SOI substrate in accordance with an embodiment.

FIG. 4 illustrates making of active device contact lines on the silicon substrate (bottom) side of SOI substrate in accordance with an embodiment.

FIG. 5 illustrates adding solder bumps to connect active device on the silicon side of SOI substrate in accordance with an embodiment.

FIG. 6 illustrates assembling electronic devices on bond pads on the silicon side of SOI substrate in accordance with an embodiment.

FIG. 7 illustrates the optical interconnect module which has active electronic device on silicon substrate side and 3D optical waveguide on silicon device side of the SOI substrate, in accordance with an embodiment.

FIG. 8 is a schematic flow chart illustrating a fabrication method for fabricating the 3D optical interconnect module in accordance with an embodiment.

FIG. 9 shows an examplenary waveguide device on one surface of a SOI substrate which has been reduced to practice in accordance with the disclosed embodiment.

FIG. 10 shows an examplenary optical interconnect module which has been reduced to practice in accordance with the disclosed embodiment.

FIG. 11 illustrates a schematic diagram of the interconnect module including the active electronics device on the top surface (silicon substrate) and the passive waveguides at the bottom surface (silicon device surface) of the SOI substrate in accordance with the disclosed embodiment.

DETAILED DESCRIPTION

The present disclosure is further described below in combination with the drawings and embodiments. It should be understood that, the specific embodiments described herein are merely used to explain the present disclosure rather than limiting the present disclosure. In addition, it should be stated that, in order to facilitate the description, merely a part of structures related to the present disclosure rather than the whole structure are illustrated in the drawings.

In various embodiments, description is made with reference to figures. However, certain embodiments may be practiced without one or more of these specific details, or in combination with other known methods and configurations. In the following description, numerous specific details are set forth, such as specific configurations, dimensions and processes, etc., in order to provide a thorough understanding of the embodiments. In other instances, well-known semiconductor processes and manufacturing techniques have not been described in particular detail in order to not unnecessarily obscure the embodiments. Reference throughout this specification to “one embodiment” means that a particular feature, structure, configuration, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrase “in one embodiment” in various places throughout this specification are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, configurations, or characteristics may be combined in any suitable manner in one or more embodiments.

The terms “above”, “over”, “to”, “between”, and “on” as used herein may refer to a relative position of one layer with respect to other layers. One layer “above”, “over”, or “on” another layer or bonded “to” or in “contact” with another layer may be directly in contact with the other layer or may have one or more intervening layers. One layer “between” layers may be directly in contact with the layers or may have one or more intervening layers.

It needs to be noted that, specific details are illustrated in the description below for fully understanding the disclosure. However, the disclosure can be implemented in other ways different from those described herein, and it may be similarly generalized by one skilled in the art without departing from the concept of the disclosure. Therefore, the disclosure will not be limited to the specific embodiments disclosed below.

Optical interconnect is a means of communication by optical fiber cables. Compared to traditional cables, optical fibers are capable of a much higher bandwidth, from 10 Gbit/s up to 100 Gbit/s. Optical communication systems often apply a vertical-cavity surface-emitting laser VCSEL for convenient configurations and easy assembling.

The vertical-cavity surface-emitting laser, or VCSEL is a type of semiconductor laser diode with laser beam emission perpendicular from the top surface, contrary to conventional edge-emitting semiconductor lasers (also in-plane lasers) which emit from surfaces formed by cleaving the individual chip out of a wafer. Vertical cavity self-emitting laser or VCSEL laser emitting light at wavelengths from 650 nm to 1300 nm are typically based on gallium arsenide (GaAs) wafers with diffraction Bragg reflectors (DBRs) formed from GaAs and aluminum gallium arsenide (Al_(x)Ga_((1-x))As).

There are now two main methods of restricting the current in a VCSEL characterized by two types of VCSELs: ion-implanted VCSELs and Oxide VCSELs. An additional adhesive layer, such as a non-conductive film (NCF), may also be applied to enhance adhesion of the components to the silicon substrate. The high speed electrical traces, including an RF transmission devices, are designed on the waveguide surface to connect the driver IC and VCSEL arrays as well as to connect trans-impedance amplifier (TIA) arrays. Typically a VCSEL array has four VCSEL lasers packed in a row.

In accordance with an embodiment, the data rate of the optical engine can be operated at 25 Giga-bits per second (Gbps) per channel, and can be extended to higher data rates such as 50 Gbps per channel. VCSEL applications include fiber optic communications, precision sensing, computer mice and laser printers.

Embodiments disclosed below describe optical interconnects and application platforms. In one aspect, the optical interconnects and platforms in accordance with embodiments may be assembled without the optical lenses and fiber ribbons commonly utilized in conventional optical interconnect assemblies. In addition, assembly time can be reduced compared to conventional techniques through use of semiconductor process technologies for the formation of transmission lines and flip chip integration of the active devices such as the driver IC chip, laser, PD, and receiver (e.g. TIA) chip. In addition, the fabrication techniques may allow for improved RF performance of the electrical signals and increased data rates of the optical interconnects. The embodiments illustrated below may be integrated as a PCB-based optical interconnect. In particular, the embodiment may be utilized to improve optical coupling with the VCSEL/PD by monolithically integrating a silicon wafer interposer with a VCSEL laser within a packaging platform like PCB, and a polymer waveguide. A metal plate maybe integrated into the silicon wafer for providing mechanical strength. The metal plate may be made of stainless steel or other compatible metal.

As shown in FIG. 1, a SOI substrate 100 is made from a silicon wafer 140, which has a bottom silicon surface 170, a top silicon surface 160 above an insulation layer 128. The insulation layer is typically a layer of silicon oxide formed by implanting oxygen into silicon wafer surface to form a well-defined SiOx layer in depth, thickness and composition. Standard Simox or deep layer implant followed by surface annealing techniques are used to prepare for the substrate. IC devices preferring low leakage or radiation protection are often fabricated on the surface 160 in IC fabrication, so top surface 160 is often referred to as device layer in semiconductor technology, although it carries a passive optical device in this invention as described below.

The silicon substrate 140 must has high transmission quality at the working wavelength of the optical interconnect device. High transmission quality includes low absorption and low scattering from the silicon wafer material or wafer surfaces at the working wavelength. For infrared light in the range of 1300 nm to several microns often used in communication applications, silicon material has low absorption. The substrate 140 has two well-polished top 160 and bottom surface 170. The top and bottom surfaces can be parallel or slightly wedged to each other for low optical reflection noise. The thickness of the SOI substrate 140 is chosen to be thin so it does not cause transmission loss optically and yet has enough strength to support photonics elements on both top and bottom surfaces during its fabrication process and its long term operation. Typically the thickness is in the range of 50 microns to 2 millimeters.

Although in many optical interconnect modules, a waveguide may be formed separately and attached to one surface of a substrate, using an adhesive layer, this application discloses a silicon waveguide fabricated inside the silicon layer above the oxide on a SOI substrate. As an integrated part of the substrate, the silicon waveguide has high transmission at interfaces and easy alignment in the optical path. Meanwhile fabrication cost from attaching waveguide is much reduced.

FIG. 2 shows a preparation step to form an optical passive path, i.e. the silicon waveguide, on the silicon top surface, a.k.a. device surface of a SOI substrate in accordance with an embodiment.

As shown in the diagram 200 in FIG. 2, a straight line 120 is patterned on the top surface of the SOI substrate 140, extra material is removed all way to the insulating layer interface 129. End portions of the line will be vertical. This patterning step is performed using lithography techniques and selective etch when silicon is removed but silicon oxide is intact.

FIG. 3 shows another preparation diagram 300 in forming two 45 degree reflectors 125 a and 125 b for the waveguide 122 on the silicon device surface of SOI substrate in accordance with an embodiment. A number of ways in controlling the lithography exposure energy can be applied to achieve smooth and accurate 45 degree slopes. For example, a moving exposure shutter or a shrinking window during photo-exposure or a variable scanner light intensity during exposure are available techniques. Reflector angle control tolerance and surface smoothness are critical to optical path efficiency, therefore fabrication of this step should have appropriate speciation requirements. Advanced micro patterning technology can provide such capability today.

Such formed waveguide has a straight portion 124 and two 45 degree sections 125 a and 125 b which serve like bending mirrors. Light travels from the other side of the substrate into the waveguide, reflects off one end to the horizontal portion 124 in the waveguide, hits the second end through total internal reflection and reflects off by the second 45 degree reflector and returns back to the substrate side of the SOI wafer. There will be no bonding step to integrate the waveguide.

Typically the SOI wafer surfaces are (100) crystalline silicon, so the silicon waveguide is fabricated in (100) silicon crystal facet, and the waveguide sidewalls are formed on (110) crystal facet.

It is recommended to seal the waveguide hermetically with a stable material having a refractive index lower than that of silicon to guarantee total internal reflection. Native oxide grown in ambient over the exposed silicon surface is about 100 nm thickness. Given that silicon's index at 1100 nm or longer is about 3.50, and oxide film has low index 1.45, a thick oxide film can be a good cladding for the waveguide, but the native oxide will not provide a good seal to stop light in vicinity of high index and high leakage materials. Therefore the waveguide should not be exposed open to ambient, instead needs to be coated with a cladding layer such as silicon oxide at least several wavelengths thick. Such option is not shown in the figures.

FIG. 4 illustrates active device contact lines on the silicon substrate side of SOI substrate in accordance with an embodiment. In the process of fabricating the SOI substrate, it is held firmly by a stage holder of some sort, preferred holding inside the area where devices are made in a contactless way. To work on packaging the active photonics devices on the substrate side of SOI, the SOI waver is flipped upside down and the passive optics side is facing down as the wafer is held on the edge by a support stage. It is optional which order is used to fabricate the two sides. As an examplenary method, waveguide side is made before the optoelectronic side. However, there is advantage of packaging the active device side first. For example, it might be more risky to process waveguides first and then hold the completed waveguide side face down, because the down facing side has more chance to touch the stage.

An insulating film 129 c is either deposited or grown on the silicon surface 170. The insulating film 129 c may include organics such as polyimide for flexibility. The surface 160 is a platform to carry interconnects for active optical devices, such as laser diodes in a transmitter or photodetectors in a receiver, electronic contacts. First, electrical traces 110 x (various transmission lines) are patterned on the clean top surface of SOI as transmission lines of the optical interconnect. Please note that in FIG. 4, transmission lines are not all drawn or labeled. Copper, aluminum, tungsten, titanium, or alloys are chosen as materials for high speed transmission lines. Metal layer deposition or other patterning techniques known in the semiconductor industry are applied in some cases. These techniques include, but not limited to, wet/dry etch, metal plating, or laser writing. Other techniques can also be applied such like, for example, first forming trenches into the top surface of the substrate; second, depositing a metal layer on the top surface to fill the trenches, followed by a planarization technique like chemical mechanical polishing (CMP) or selective dry/wet etching to remove the excess metal from outside the trench areas.

FIG. 5 illustrates adding solder bumps in order to connect active devices on the silicon side of SOI substrate in accordance with an embodiment.

After patterning transmission lines 110 x on the top surface, proper bonding pillars 141 x are formed on the transmission lines 110 x as contacting spots for connecting to active optical devices or other electronics. Again not all bonding bumps are shown as 141 x in FIG. 5. The connection from this substrate interposer to external circuits may be achieved by a variety of semiconductor packaging techniques. For example, the transmission lines and bond pads can be defined by lithography process.

A metal plate may be included in the substrate to strengthen the interposer. The metal plate maybe made of compatible materials such as stainless steel.

The SOI substrate forms a mechanical support structure for a PCB board, silicon waveguide or waveguides in a multi-channel system, and a connector to an optical fiber cable. Without the SOI substrate as an interposer, a film based PCB maybe too fragile to support the electronic devices and waveguides.

FIG. 6 illustrates an examplenary schematic diagram 600 of electronic devices on bond pads assembled on the bottom silicon surface of SOI substrate in accordance with an embodiment.

Active optical devices in this system such as vertical cavity surface emission laser (VCSEL) 121, laser driver 121A, photodiode (PD) 127, trans-impedance amplifier (TIA not shown in the figure), RF circuits, and other electronic devices are attached to transmission lines 110 x engraved as metal-in-trench structures on bottom surface of the SOI substrate. Other optoelectronic devices maybe assembled on a PCB unit packaged to wafer surface. Direct patterning transmission lines on the surface enables high speed performance.

An adhesion film maybe deposited on the surface over the transmission lines 110 x and interconnecting posts 141 x to passivate and insulate them from the active devices which are arranged above. Another way to insulate maybe to apply a non-conductive film (NCF), over the top surface followed by heating to enhance adhesion of the parts. A cleaning process is also applied to expose the bonding pillars 141 x to properly connect to the active devices. The adhesion film or NCF is not shown in FIG. 6.

The above described active optoelectronics assembly including the optoelectronic connections forms an optical engine. The optical engine is assembled to external circuits packaged through bond pads on a PCB 150.

Another option for this interconnect from transmission lines to active devices is through an optoelectronic packaging technique such as the anisotropic conductive film (ACF), not shown in FIG. 6.

ACF technology is widely used in optoelectronic packaging for higher signal densities and smaller overall packages. In this process, the anisotropic material, for example, a thermosetting resin containing conductive particles, is first deposited on the PCB 150 using a lamination process. The optical engine on the SOI substrate is then placed in position over the PCB and the two sides are pressed together to mount the optical engine to the PCB board. In many cases this mounting process is done with no heat or a minimal amount of heat that is just sufficient to cause the anisotropic material to become slightly tacky. In using a thermosetting resin containing conductive particles, the particles are trapped between the PCB and the optoelectronics elements, thereby creating an electrical connection there between. In other locations where it is not terminated with electrodes, particles are insulated by the thermosetting resin. For bonding, the amount of thermal energy required is higher due to the need to first flow the adhesive and allow the two sides to come together into electrical contact, and then to cure the adhesive and create a lasting reliable bond. The temperatures, times, and pressure required for these processes must be controlled properly.

The active devices, such as (VCSEL) 121 or VCSEL array (for example a standard 1×4 VCSEL), photodetector (PD) 127, receiver chip or trans-impedance-amplifier (TIA) arrays, and in addition, driver IC 121A, and RF device, are flip chip assembled to the bonding pillars 141 x on the bottom side 129 c of the SOI substrate 140 to form the optical engine. The high speed transmission lines 110 x, and the RF device, are therefore designed on the surface 129 c to connect the driver IC 121A to VCSEL 121 as well as to connect the receiver chip TIA arrays and other electrical lines. In accordance with the embodiments, formation of the electrical traces 110 x on the substrate may lead to improved RF signals performance from the high dielectric constant of the silicon material, typically ranging in 3-4 for infrared light. The data rate of the optical engine can be operated at 25 Giga-bits per second (Gbps) per channel, and can be extended to higher data rates such as 50 Gbps per channel.

In some cases, the optical engine is assembled with a flexible printed circuit board (FCB). In an embodiment, the optical engine is assembled within a periphery of an opening in the PCB. While not separately illustrated, such a configuration is compatible with other embodiments, including that described with regard to FIG. 6.

FIG. 7 illustrates the optical interconnect module which has active electronic device on silicon substrate side and 3D optical waveguide on silicon device side of the SOI substrate, in accordance with an embodiment.

In FIG. 7, VCSEL laser output is aligned to match the first 45 degree reflector 125 a through the SOI substrate along path 123 a, a 90 degree bend couples light to the straight portion of waveguides 122. Light is reflected the second time by the second 45 degree reflector 125 b, back into photodetector 127 through the SOI substrate the second time to complete the full optical path as shown in FIG. 7. The device side of the SOI only supports the passive optical waveguide devices, and the transceiver and receiver are all kept on the opposite SOI bottom side.

The 45 degree reflectors are introduced in a silicon waveguide to form a 3D optical path on a single chip, which enables a compact configuration in on-chip optical interconnect application.

FIG. 8 is a schematic flow chart illustrating a fabrication method for fabricating the 3D optical interconnect module in accordance with an embodiment. The examplenary fabrication method proposes to make the active device side first and the passive waveguide side afterwards.

The fabrication sequence may include the first step 802: provide a SOI substrate having a Si surface for optoelectronics and a Si-on-insulator surface for waveguides. The two surfaces may be parallel or slightly wedged to each other to avoid problems caused by ghost reflections. In step 804, deposit an isolation layer on top surface to isolate the silicon from the metal lines. In step 806, pattern trenches on top surface and fill in metal to form transmission lines, or deposit metal layer and pattern it into lines with etching Laser writing is also a way to get lines formed. The metal layer may be aluminum, tungsten, etc. If it is copper, process will need to follow copper plating techniques. In step 808, grow bonding pillars or solder bumps with gold or nickel coating for flip chip mounting with bond pads. In step 810, assemble active optical devices (VCSEL/PD) with transmission lines via interconnecting pillars, solder bumps. In step 812, deposit an anisotropic conductive film (ACF) on top surface (optional). In step 814, attach PCB to pads or ACF. In step 816, turn substrate over to work on making waveguides on bottom surface. In step 818, pattern strip waveguides on si-on-insulator surface and expose oxide surface outside the waveguide area. In step 820, pattern 45 degree reflectors at both ends of each silicon waveguide, reflectors are aligned with active optical devices VCSEL and PD.

FIG. 9 shows an examplenary waveguide array device 900 on surface of a SOI substrate which has been reduced to practice in accordance with the disclosed embodiment.

FIG. 10 shows an examplenary optical interconnect module 1000 which has been reduced to practice in accordance with the disclosed embodiment. The SOI substrate top surface has on-chip transducer module VSCEL and its driver IC on the left, on-chip receiver module photo detector PD and amplifier TIA on the right, and the waveguides are mounted at the other side of the SOI substrate, not visible from the top view.

FIG. 11 illustrates a schematic diagram of the interconnect module including the on-chip active TX Driver IC and VSCEL on the left and the on-chip RX Driver IC and PD on the right on the top surface (silicon substrate), connected by the passive waveguide at the bottom surface (silicon device surface) of the SOI substrate in accordance with the disclosed embodiment. Waveguide has four channels, each has two ends receiving and reflecting light from VCSEL to PD.

However, embodiments are not limited to this particular sequence, and alternative fabrication sequences are envisioned.

In utilizing the various aspects of the embodiments, it would become apparent to one skilled in the art that combinations or variations of the above embodiments are possible for fabricating optical interconnects. Although the embodiments have been described in language specific to structural features and/or methodological acts, it is to be understood that the appended claims are not necessarily limited to the specific features or acts described. The specific features and acts disclosed are instead to be understood as embodiments of the claims useful for illustration. 

What is claimed is:
 1. An optical interconnect apparatus, comprising: a silicon substrate having a first surface and a second surface substantially parallel to each other, wherein the second surface comprises an oxide layer underneath a top silicon surface; an insulating layer deposited on the first surface; an optical engine mounted on the insulating layer, wherein the optical engine comprises: a plurality of conductive lines patterned on the first surface; an active optical device, a driver chip, and a receiver device bonded with the plurality of conductive lines; and a silicon waveguide device fabricated in the silicon and directly on the oxide layer on the second surface, wherein the silicon waveguide device comprises multiple channel waveguides each has a straight portion terminated with two 45 degree reflectors at two ends; wherein an optical path is formed from the active optical device on the first surface of the silicon substrate through the silicon waveguide device back to the receiver device on the first surface, via the help of the two reflectors.
 2. The optical interconnect apparatus of claim 1, wherein the 45 degree reflectors at two ends of each channel of the silicon waveguide device are aligned optically to the active device and the receiver device respectively.
 3. The optical interconnect apparatus of claim 1, wherein the silicon waveguide device is fabricated in (100) silicon crystal facet of the second surface, and the waveguide sidewalls are formed on (110) crystal facet.
 4. The optical interconnect apparatus of claim 1, wherein a layer of oxide is grown on the silicon waveguide device to form a cladding structure.
 5. The optical interconnect apparatus of claim 1, wherein the active optical device is a vertical cavity surface emission laser (VCSEL), or a vertical cavity surface emission laser array (VCSELs) emitting infrared light.
 6. The optical interconnect apparatus of claim 1, wherein the receiver device is a photodiode (PD) or a photodiode array (PDs), wherein the photodiode or photodiode array is a hybrid type of silicon and germanium.
 7. The optical interconnect apparatus of claim 1, wherein the optical engine further comprises amplifiers and RF circuitry.
 8. The optical interconnect apparatus of claim 1, wherein the silicon substrate has a thickness ranging from 50 microns to 2 mm.
 9. The optical interconnect apparatus of claim 1, wherein the silicon substrate is a SOT wafer.
 10. The optical interconnect apparatus of claim 1, wherein the substantially parallel first and second surfaces of the silicon substrate are wedged with an angle smaller than 5 degrees.
 11. The optical interconnect apparatus of claim 1, wherein the optical engine and the conductive lines are connected by solder bumps and wherein an adhesive layer on top of the conductive lines attach the optical engine to the silicon substrate.
 12. The optical interconnect apparatus of claim 1, wherein the second surface has a silicon layer of a thickness of 4 microns to 100 microns and wherein the oxide layer is thicker than 5 microns
 13. The optical interconnect apparatus of claim 5, wherein the vertical cavity surface emission laser array (VCSELs) is a 1×4 array and the silicon waveguide device has 4 channels.
 14. The optical interconnect apparatus of claim 1, wherein the conductive lines comprise tungsten (W), aluminum (Al), copper (Cu), gold (Au), silver (Ag), stainless steel, or an alloy.
 15. The optical interconnect apparatus of claim 1, wherein the conductive lines are high speed RF transmission lines capable of operating at 25 Gbps per channel.
 16. The optical interconnect apparatus of claim 1, wherein the active optical device is a vertical cavity surface emission laser (VCSEL), or a vertical cavity surface emission laser array (VCSELs) emitting visible light.
 17. A method of fabricating an optical interconnect apparatus, comprising: providing a silicon substrate having two substantially parallel first and second surfaces: depositing an insulating layer on the first surface; patterning a plurality of conductive lines on the insulating layer; growing bonding pillars or solder bumps and coating with Au/Ni for flip chip to mount active optical devices such as a vertical cavity surface emitting laser (VCSEL) and a photodetector (PD); turning the silicon substrate over to work on second surface; patterning a straight portion of the silicon waveguide on the second silicon surface; patterning two 45 degree end reflectors on the straight portion of the silicon waveguide, wherein the patterning comprises variable exposure intensity in lithography process, wherein the two 45 degree end reflectors are aligned with active optical devices VCSEL and PD; removing excess silicon outside the silicon waveguide to expose the oxide layer; and depositing a layer of oxide on the waveguide.
 18. A method of fabricating an optical interconnect apparatus in claim 17, wherein depositing an adhesive layer on the first surface including providing an anisotropic conductive film (ACF).
 19. The method of fabricating an optical interconnect apparatus in claim 17, wherein patterning the plurality of conductive lines comprises forming trenches and filling in metal in the trenches, followed by removing excess metal by polishing (CMP) or by selective etch to clean up metal outside the trenches.
 20. The method of fabricating an optical interconnect apparatus in claim 17, wherein patterning the plurality of conductive lines comprises depositing a metal layer, patterning the metal layer into lines, followed by removing excess metal by polishing (CMP) or by selective etch. 